1. Field of the Invention
The present invention concerns a method of mapping chips of a spread out symbol to sub-channels of a group of sub-channels of a Multi-Carrier transmission network.
2. Discussion of the Background
In a Multi-Carrier transmission network using spreading sequences for multiplying data items to be transmitted, such as a Multi-Carrier Code Division Multiple Access transmission network, better known by the name MC-CDMA transmission network or an Orthogonal Frequency and Code Division Multiplexing transmission network, better known by the name OFCDM transmission network, the L chips of each spread out symbol formed by multiplying a data item to be transmitted by L elements of the spreading sequence are mapped to L sub-channels of the Multi-carrier transmission network before an Orthogonal Frequency Division Multiplexing modulation and their transmission on the transmission network.
OFCDM and MC-CDMA transmission networks use orthogonal spreading sequences in order to selectively transmit and/or receive data to and/or from each of the users of the transmission network. OFCDM an MC-CDMA transmission networks suffer from multiple access interferences due to a loss of orthogonality among user's signals after their propagation through the multiple path channel. Since the channel is selective in frequency due to multi-paths and in time due to Doppler variations, the orthogonality among user's signals may be corrupted.
In order to reduce multiple access interferences, it has been proposed to optimise the selection of spreading sequences required to the different users.
The chip mapping also, aims at reducing the effect of the Multi-Carrier transmission channel selectivity by defining groups of sub-channels in the Multi-Carrier transmission channel that are correlated. The channel selectivity can be decomposed into two parts, the channel frequency selectivity and the channel time selectivity.
The channel frequency selectivity results from multi-path propagations. Obstructions generated by houses and other obstacles located between the transmitter and the receiver make that the transmitted signal is propagated on multiple paths, each path being delayed and attenuated differently. As a result, the signals from multiple paths arrive at the receiver at different times and these signals added constructively and destructively produce signal fading.
The channel time selectivity results from Doppler shifts due to the fact that the receiver or the transmitter or obstacles along the transmission path are moving. The channel can be then described as a time varying response channel.
In order to avoid problems generated by the channel frequency selectivity, it as been proposed to map the L chips of the spread out symbol on L several consecutive sub-carriers of the same time slot. Such technique is called a one dimensional frequency domain spreading.
In order to avoid problems generated by the channel time selectivity, it as been also proposed to map the L chips of the spread out symbol on L several consecutive time slots at the same frequency. Such technique is called a one dimensional time domain spreading.
It has been also proposed to combine both of these one dimensional spreading techniques and then to realize a two-dimensional in time and frequency domain spreading. According to such technique, the L chips of the spread out symbol are mapped on L sub-channels involving consecutive sub-carriers and consecutive time slots. Thanks to that technique, it is then possible to limit problems generated either by the channel frequency selectivity and the channel time selectivity.
Such two-dimensional spreading is disclosed in the presentation documents to the International Forum on fourth Generation Mobile Communications entitled “Broadband Packet Wireless Access and its Experiments” by Hiroyuki Atarashi. In that document, the proposed mapping is not optimum in the sense that some large de-correlation may occur for some sub-channels that are mapped to consecutive chips of a spread out symbol.